In traditional (analog) screen/film radiography, the film functions both as a recording medium, converting light from the screen into a latent image that is rendered visible after chemical processing, and as a display medium on a viewbox. Therefore, the characteristic curve of the screen/film system largely determines the contrast with which small and low-contrast details are displayed in the output image (film). In order to provide the best output image, the radiographer must try to control the amount of scatter radiation reaching the film. The principal factors that affect the amount of scatter produced are the kilovoltage applied to the radiation generator and the irradiated material (e.g., tissue). As kilovoltage increases, the percentage of primary photons that will undergo scattering also increases. As the volume of irradiated tissue increases, the amount of scatter produced is increased. Volume will increase as the irradiation field size increases or as the patient thickness increases. The atomic number of the irradiated material also has an impact on the amount of scatter produced. Higher atomic number materials have a greater number of electrons within each atom, and therefore photons have a greater chance of interacting with these materials.
To control the amount of scatter and reduce patient dose, one of the most common methods is the use of some beam-restricting devices such as aperture diaphragms, cones/cylinders and collimators. Of them, the collimator is the most commonly employed beam restrictor in radiography because it permits an infinite number of field sizes using only one device. It also has the advantage of providing a light source for the radiographer as an aid in properly placing the x-ray source tube. Accurate collimation of the x-ray beam to the region of interest reduces the area and volume of tissue irradiated and provides for a reduced amount of scatter reaching the film, resulting in better image contrast.
In digital radiography, on the other hand, storage phosphors are used for the digital acquisition of projection radiography. These phosphors offer a very wide exposure latitude (10.sup.4 :1) compared with that (40:1) of the traditional screen/film radiography. Because of this wide range of detectable exposures, the necessity of re-exposing a patient due to improper selection of exposure factors is virtually eliminated. Moreover, the separation of image acquisition and display stages provides opportunities for the electronic processing, storage, and transmission of radiographic images.
In the descriptions that follow, the irradiation field will be used to denote the image area containing the body part and the direct x-ray exposed region which has received unattenuated x-rays. The non-irradiation field will be used to denote the very low intensity area, wherein a highly absorbent beam restrictor is used to "frame" the irradiation field in the acquired image.
Although image processing techniques are allowed to be applied in digital radiography, the effectiveness of such techniques depends on the careful choice of the various parameters that control their performance. For example, histogram-based tone-scale transformation is a simple and effective way of adjusting the contrast of an image. However, the histogram is a global characteristic of the image, and therefore does not distinguish between the anatomically important regions of the image (e.g., the body part) and the unimportant regions of the image (e.g., the non-irradiation field). Thus, a tone-scale transformation based on such a histogram will be suboptimum if it is unduly influenced by the unimportant regions of the image.
Due to the importance of the information from the irradiation field, a variety of prior art methods have been proposed to detect the irradiation field of a radiographic image. For example, U.S. Pat. No. 4,731,863 teaches a technique for finding gray level thresholds between anatomical structures and image regions based on zero-crossings of a peak detection function derived from application of smoothing and differencing operators to the image histogram. This method produces a series of peaks by analyzing the histogram at several different signal resolutions. According to this method, the peaks need to be interpreted for each exam type and exposure technique. That is, for one exam type or exposure technique a low-signal peak could correspond to a non-irradiation field, but for another type or technique it could also correspond to a body part if no non-irradiation field is present in the image. Thus, some additional information may be needed to complete the analysis.
Other methods of histogram analysis have also been proposed. U.S. Pat. No. 4,952,805 teaches an irradiation field finding technique based on dividing the histogram into several sections with an intensity thresholding procedure and then doing a statistical shape analysis (discriminate analysis) of the section believed to correspond to an irradiation field. A decision about the presence of an irradiation field is made based on the shape of this section in the histogram. However, the large variety of histogram shapes that can occur with different exam types and different input modalities (such as magnetic resonance imaging (MRI), computed tomography (CT), ultrasound (US), nuclear medicine, digital subtraction angiography (DSA), and computed radiography (CR)) make this type of analysis subject to error. In addition, since a single threshold is chosen to represent the transition from a non-irradiation field to an irradiation field, this method does not perform well when the transition is fairly wide, such as when x-ray scatter is present.
European Patent Application 288,042, published Oct. 26, 1988, proposes an irradiation field finding method using the image histogram. In this method, the histogram is again divided into a number of sections by an automatic thresholding procedure. Then a statistical analysis (discriminate analysis), combined with information about the exam type, exposure technique, and desired body portion to be displayed, is used to adjust the separation points between the sections until desired ranges for the irradiation field are found. This method is less prone to variations in exam type and input modality because this information is incorporated into the decision process. However, the se of fixed thresholds still poses problems if there is nonuniformity in the non-irradiation field.
A more effective way of detecting the irradiation field of a radiographic image is to include spatial information in the analysis, in addition to the intensity information provided by the histogram. Several methods have been described for doing this. U.S. Pat. Nos. 4,804,842 and 5,028,782 disclose a method for detecting the irradiation field in an image based on calculating derivatives of the input image and then identifying those points whose derivatives are higher than a threshold value identified with edge points. Then a new histogram of the input image is done using only the points identified as edge points, and from this histogram another threshold value is chosen to represent the boundary of the irradiation field. This method is claimed to provide a more accurate measure of the field than a simple histogram method. However, it still requires a priori knowledge that a collimator or field stop was in fact used to define the irradiation field, otherwise low-signal portions of the image inside the body part may be clipped by the intensity thresholding that defines the boundary. Furthermore, if image pixels inside the body part have a signal value comparable to or lower than those in the non-irradiation field (as when there is significant x-ray scatter), the edge of the irradiation field may not even be found with this method. Finally, if the region in the non-irradiation field is nonuniform in intensity, which is frequently the case when there is scatter present, there will not be a strong edge at the boundary of the irradiation field, and the derivative at the edge points may not have a high enough value to pass the threshold, leading to inaccuracies in finding the edge points.
Other irradiation field detection methods have been described that use one dimensional edge detection along arbitrary lines drawn across the image. For example, U.S. Pat. No. 4,967,079 discloses a method for storage phosphor digital radiography systems that uses derivatives along radial lines from the image center, followed by a thresholding operation to detect potential edge points of the irradiation field. The boundary of the field is recognized by testing the colinearity of the found edge points. In order to be effective, this method requires a strong edge transition from the non-irradiation field to the irradiation field. However, the transition can sometimes be very weak and even inverted (body part with a lower signal than foreground, due to scatter). Furthermore, if the image involves multiple smaller images recorded on a large detector (so-called subdivision or multiple exposure recording), there will be many edges detected along radial lines from the image center, possibly leading to the detection of false boundaries.
An alternate approach to irradiation field detection has been disclosed in U.S. Pat. No. 4,859,850. In this case, lines are extended from the edge of the image towards the center and, for each line, the transition regions from low signal to high signal at the edge of an irradiation field are fit with a linear or nonlinear equation. When the differences between the extrapolated fitted values (calculated from the equation) and the actual image values inside the field become too large, or when the extrapolated values reach a threshold signal level, the edge of the field is assumed to have been found. One problem with this method is that it assumes that a non-irradiation field has to be present (i.e., a priori knowledge of the exposure technique is required). A second problem has already been mentioned above, namely, that the method assumes that the signal values inside the irradiation field are always larger than those immediately outside it, which is not always the case when scatter is present. A third problem is that if subdivision recording has been used, the method may not find all of the necessary edges to define each irradiation subfield within the image. Finally, the use of multiple linear or nonlinear fits on multiple lines across the image is an inefficient, time-consuming way to find the irradiation field boundaries.
A possible solution to the previously mentioned problem of detecting edges in subdivision recording has been proposed in U.S. Pat. No. 4,851,678. In this method, designed for storage phosphor digital radiography, potential edge points can still be found using the above method of differentiation along lines, but other possibilities are also disclosed. For example, once a few candidate edge points have been found, a boundary tracking procedure, based on following the likeliest edge points around the boundary from nearest neighbor to nearest neighbor (using a ridge-following algorithm) until they close on themselves again, is used to find the irradiation field. This method purports to handle multiple exposure subfields as well because multiple starting edge points can be followed around their respective irradiation subfields. Since the method of finding prospective edge points is similar to those above, similar potential problems exist, namely, that the method can break down when the edge transition from the irradiation field to the non-irradiation field is weak. Also the ridge-following algorithm can be very sensitive to noise, so the image data must be smoothed before analysis.
As indicated above, the presence of multiple smaller images recorded on a single larger recording medium (i.e., subdivision recording) can create problems in locating all of the irradiation subfields in the image. Sometimes a preprocessing stage can be used to identify the use of subdivision recording and also the format of the image (2-on-1, 4-on-1, etc.). For example, U.S. Pat. No. 4,829,181 teaches a method of recognizing a subdivision pattern in a storage phosphor system using differentiation to detect prospective edge points, followed by a colinearity test to see if the edge points lie on straight lines. If the edge points lie on straight lines, subdivision recording is judged to be present. A limitation of this method is that it can detect only rectilinear patterns, i.e., patterns with essentially horizontal or vertical linear separations.
Another approach to detecting such subdivision patterns is the use of pattern matching. U.S. Pat. No. 4,962,539 discloses a method that uses a set of binary, stored masks representing typical subdivision recording patterns. The input digital image is converted into a binary image by thresholding, and the resulting binary image is statistically compared with each of the masks in the stored set. The stored mask with the highest degree of matching is judged to be the recording pattern on the input image. While this method can handle a wider variety of patterns than the one above, it is still limited to the stored library of patterns for matching. Any irregular patterns not included in the library may not have a high degree of matching, and may therefore be chosen incorrectly. Furthermore, statistical matching is complex and time consuming.
In another prior art method, U.S. Pat. No. 5,268,967 discloses a method that first analyzes the edge content of the image, and then breaks the image into a set of nonoverlapping, contiguous blocks of pixels. The edge density in each block is computed as one indicator of the level of detail or "busyness" in various locations of the image, and, therefore, an indicator of whether these locations are more likely to belong to the irradiation or non-irradiation field.
Further analysis of the image and classification into the various regions take place on a block-by-block basis. Although the reliability of this method is good, in particular for single exposure images, the level of complexity is high and the speed is prohibitive for real-time applications.
Thus a need remains for an automated method and apparatus for digital image processing to perform the detection of an irradiation field and multiple irradiation subfields. Such a method and apparatus would allow the parameters of subsequent image processing techniques to be calculated more robustly and efficiently, leading to better image quality and more accurate diagnosis.